// RotatedTriangle.js (c) 2012 matsuda
// Vertex shader program
var VSHADER_SOURCE =
    // x' = x cosβ - y sinβ
    // y' = x sinβ + y cosβ　Equation 3.3
    // z' = z
    `attribute vec4 a_Position;
    uniform mat4 u_xformMatrix;
    void main(){
        gl_Position=u_xformMatrix *a_Position;
    }`;

// Fragment shader program
var FSHADER_SOURCE =
    `void main() {
      gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
    }`;

var ANGLE_STEP = 45.0;

function main() {
    // Retrieve <canvas> element
    var canvas = document.getElementById('webgl');

    // Get the rendering context for WebGL
    var gl = getWebGLContext(canvas);
    if (!gl) {
        console.log('Failed to get the rendering context for WebGL');
        return;
    }

    // Initialize shaders
    if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
        console.log('Failed to intialize shaders.');
        return;
    }

    // Write the positions of vertices to a vertex shader
    var n = initVertexBuffers(gl);
    if (n < 0) {
        console.log('Failed to set the positions of the vertices');
        return;
    }


    var u_xformMatrix = gl.getUniformLocation(gl.program, 'u_xformMatrix');
    if (u_xformMatrix < 0) {
        console.log('Failed to get the storage location of u_xformMatrix');
        return;
    }

    // Specify the color for clearing <canvas>
    gl.clearColor(0, 0, 0, 1);

    var currentAngle = 0.0;
    var xformMatrix = new Matrix4();

    var tick = function() {
        currentAngle = animate(currentAngle);
        draw(gl, n, currentAngle, xformMatrix, u_xformMatrix);
        requestAnimationFrame(tick);
    };
    tick();


}

function initVertexBuffers(gl) {
    var vertices = new Float32Array([
        0, 0.5, -0.5, -0.5, 0.5, -0.5
    ]);
    var n = 3; // The number of vertices

    // Create a buffer object
    var vertexBuffer = gl.createBuffer();
    if (!vertexBuffer) {
        console.log('Failed to create the buffer object');
        return -1;
    }

    // Bind the buffer object to target
    gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
    // Write date into the buffer object
    gl.bufferData(gl.ARRAY_BUFFER, vertices, gl.STATIC_DRAW);

    var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
    if (a_Position < 0) {
        console.log('Failed to get the storage location of a_Position');
        return -1;
    }
    // Assign the buffer object to a_Position variable
    gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 0, 0);

    // Enable the assignment to a_Position variable
    gl.enableVertexAttribArray(a_Position);

    return n;
}

function draw(gl, n, currentAngle, xformMatrix, u_xformMatrix) {
    xformMatrix.setRotate(currentAngle, 0, 0, 1);
    gl.uniformMatrix4fv(u_xformMatrix, false, xformMatrix.elements);
    // Clear <canvas>
    gl.clear(gl.COLOR_BUFFER_BIT);

    // Draw the rectangle
    gl.drawArrays(gl.TRIANGLES, 0, n);
}
var g_last = Date.now();

function animate(angle) {
    var now = Date.now();
    var elapsed = now - g_last;
    g_last = now;
    var newAngle = angle + (ANGLE_STEP * elapsed) / 1000.0;
    return newAngle %= 360;
}